A specific application of the invention is in the monitoring of a Butler matrix-fed circular phased array antenna system. A general description of a circular phased array antenna system and the theory of operation thereof is contained in a paper titled: "A Matrix-Fed Circular Array for Continuous Scanning" by B. Sheleg, Proc. IEEE, V. 56, no. 11, (Nov. 1968). The Sheleg reference makes no mention of a monitoring system for such an antenna.
This invention is an improvement upon the monitoring system disclosed in U.S. Pat. No. 4,639,732, issued Jan. 27, 1987, to J. Acoraci and A. Moeller for "Integral Monitoring System for Circular Phased Array Antenna", and assigned to the assignee of the present invention.
The circular phased array antenna described in the Acoraci et al. patent comprises sixty-four pairs of dipole radiating elements, with the dipole pairs arranged vertically and evenly spaced about the circumference of a cylindrical ground plane. Each dipole pair is fed energy from one of sixty-four output ports of a Butler-type beam forming matrix. The antenna system further includes a plurality of digital phase shifters, one for each excited input mode of the Butler matrix, which permit fine steering of the array beam to any selected one of 1024 evenly spaced azimuth radials.
The monitoring system disclosed in the Acoraci et al. patent utilizes four independent monitor signal circuits, one for each of the four quadrants, which are spaced around the circumference of the array. Each monitor signal circuit includes an r.f. monitor assembly which spans one-quarter of the circumference of the array. Each r.f. monitor assembly includes sixteen probes, one for each of the dipole pairs in a quadrant of the array, that are each connected to a common transmission line through individual fixed phase shifters and couplers. Each of the probes is located in near proximity to a dipole pair. The fixed phase shifters and couplers associated with the probes are so designed that the signal output from the common transmission line simulates the signal that would be received by an antenna positioned in the far field of the array along the 45.degree. radial of the quadrant covered by the r.f. monitor assembly.
The monitoring system of Acoraci et al. operates during the normal transmit mode of the antenna system. As the beam of the array is scanned in azimuth, the amplitude of the signal output of the monitor circuit for the quadrant in which the beam is then located is compared with stored values of signal output previously obtained from a fully functional array. Such a comparison is made at each of the 256 beam positions within a quadrant. If the comparison shows a departure in the signal output by more than a tolerable amount at one or more of the beam positions within a quadrant, a fault signal is generated, indicating a failure at one or more of the sixteen dipole pairs within that quadrant. It is then necessary, using other procedures, to test individually each of the dipole pairs of that quadrant to identify the particular one or ones of the dipole pairs at fault.
It is an object of the present invention to provide a method and means for monitoring a phased array antenna system to provide a warning of the presence of faulty components in the radiating elements of the antenna system.
It is a more particular object of the invention to provide a method and means for monitoring a circular phased array antenna system comprised of a plurality of columns of radiating elements to provide a warning of the presence of faulty radiating columns in the system and to provide an indication of the particular one or ones of such columns at fault.
Other objects and advantages of the invention will become evident as an understanding thereof is gained from the following complete description and the accompanying drawings.